To determine the moisture content of a sample, the sample is dried and its weight is determined before and after the drying process.
Under certain circumstances, the weight loss may also be measured during the drying process. In this case, the weight value for a sample decreases as a function of the temperature, the drying time, and the conditions in the test chamber, wherein the weight value follows a weight-time curve that asymptotically approaches the dry weight of the sample. In order to test whether the moisture of a sample is equivalent to a predefined nominal value, the weight-time curve of the sample is calculated. Then, the weight-time curve calculated in this manner is compared with a comparison curve that is determined in comparison experiments. The comparison curve is based on a weight-time curve that has been obtained using a sample whose moisture is equivalent to the nominal value. If the calculated weight-time curve and the comparison curve do not match within predefined tolerances, the moisture of the sample is equivalent to the nominal value within the predefined tolerance value.
An appropriately equipped measuring instrument for gravimetric moisture determination is able to measure the moisture content of a sample on the basis of the measured curve parameters and the drying time, and to display such values on a display unit using existing electronic means, which include associated computing and storage capability. With this method, the material to be dried no longer has to be completely dried out, it is sufficient to determine the coordinates of two detection points in the weight-time diagram.
The drying process in which the material to be dried is not completely dried out may be conducted for a predefined, constant drying time Δt. Alternatively, the drying process may be continued until the change in mass per unit of time in falls below a predefined threshold value a during the drying process, wherein the drying time Δt does not necessarily have to be constant.
As noted, the change in weight of a sample during the drying process depends essentially on the temperature, the drying time, and the conditions in the test chamber. A chamber that may be opened to allow the deposit or removal of a sample, and which is enclosed by the housing of the measuring instrument serves as the test chamber. An arrangement for holding the sample and a means for heating the sample are also located inside the test chamber. The sample holder is connected to a gravimetric measuring instrument.
Normally, a thin film of the sample is deposited on a flat sample holder, for example a sample tray. The tray is arranged in the measuring instrument for gravimetric moisture, preferably with its surface area horizontal and parallel with respect to the surface area of means for heating the sample, to enable the sample to be heated evenly.
Various radiation sources, such as radiant heaters, ceramic heaters, halogen lamps and quartz lamps are used as the heating means. It is even possible to use microwave generators as a source of radiant heat.
A measuring instrument for gravimetric moisture determination of the species described is known for example from European patent specification EP 0 611 956 B1. In this device the sample holder is loaded outside of the measuring instrument for gravimetric moisture determination. To do this, the weighing device is extracted from the measuring instrument housing together with the sample holder on a pull-out device in the form of a drawer. An annular halogen lamp is used as the radiation source, and is located above the sample holder when in operation.
Another measuring instrument is known from European patent application EP 1 850 110 A1. In this device, two radiation sources are used, and in the measurement position the sample holder is arranged between the first and the second radiation sources.
To assure that the measuring instrument for gravimetric moisture determination is functioning precisely, it must be tested regularly, and adjusted or calibrated as necessary. One option is to test the weighing device and the radiation source separately from one another. In order to test the weighing device, a known reference mass can be placed on the mass sensor. If the mass displayed by the measuring instrument differs excessively from the reference mass, the mass sensor must be readjusted. In order to test the radiation source, a reference temperature sensor can be inserted in the measuring instrument instead of the sample. The measuring instrument is then heated to a predefined reference temperature. If the temperature displayed by the measuring instrument differs excessively from the reference temperature, the radiation source must be readjusted. Reference temperature sensors are known for example from German patent DE 100 24 015 C2.
Testing of the radiation source is particularly time-intensive, since the reference temperature sensor must be inserted in the cold measuring instrument in order to obtain the best accuracy possible. In this context, the measuring instrument is understood to be cold if it is at room temperature, approximately 20° C. It is also necessary to wait until the instrument has reached a stable state and the reference temperature sensor does not display any temperature fluctuations, and this too takes a great deal of time. In order to ensure that the radiation source is absolutely reliable, it is further recommended to carry out this test for more than just one reference temperature. As a consequence, the test requires significant effort, and is therefore often performed irregularly by the user, which in the meantime may also lead to uncertainties regarding measurement accuracy.
A test of the reproducibility of measurement results is described in published application US 2002/063128 A1. For this test, milk powder from five different manufacturers is used. Two samples of the milk powder from each manufacturer are taken, and the moisture content in each of these samples is determined. The measurement results for two corresponding samples may be compared. Conclusions regarding the reproducibility of the measurement results may be drawn on the basis of these measurements. However, no conclusions can be drawn regarding the absolute accuracy of the procedure. If the measurement device were to show an incorrect value, this would lead to a systematic error. The measured values recorded with the faulty measurement device would be the same.
In another analysis method, which is also lengthy, however, the moisture of a test substance is determined using a Karl Fischer titration procedure. Then, a comparison sample is taken from the test substance and the moisture of the comparison sample is determined using the measuring instrument. If the measuring instrument returns a sufficiently accurate result, within the parameters of permissible error tolerances, the measuring instrument does not need to be adjusted or calibrated again. However, if the result is outside the permissible error tolerances, the measuring instrument must be adjusted and/or calibrated.
The drawback of this analysis method too is that determining the moisture of the test substance by Karl Fischer titration is a lengthy process and consequently is often carried out only sporadically by the user, and here too this can lead to uncertainties regarding the accuracy of the measurement.
WO 99/61878 A and US 2002/063128 A1 disclose other analysis methods, which are also time consuming. In these methods, the moisture of a test substance is determined using a standard convection oven. For this purpose, a sample of the liquid-containing test substance is first weighed, and then dried in the convection oven. After drying, the sample is weighed again. The moisture content of the test substance can be determined from the weight difference of the sample before and after it is dried. This analysis method also suffers from the drawback that determining the moisture of the test substance is time consuming and therefore is often not carried out regularly by the user.
The object, then, is to provide a method for testing a measuring instrument for gravimetric moisture determination, and a suitable test sample therefore, enabling the measuring instrument to be tested as simply and accurately as possible.